It is standard practice to ship and store spent nuclear-reactor fuel rods in large metallic containers formed normally of vessels and covers both made of spherulitic cast iron or even steel. Such a container is quite large, having wall thickness of 0.2 m to 0.6 m and an overall height of several meters. The vessel can be made as described in copending patent application Ser. No. 379,890 filed 5/1982 of Friedrich Werner, and may have inclusions of shielding metal such as lead or even lead bars imbedded in its walls.
The cover of such a container is formed with a plug that fits within the mouth of the vessel. For best sealing action the vessel mouth and plug are complementarily formed with at least one interfitting shoulder bordered by an annular nonplanar--usually cylindrical or frustoconical--surface. Seals, typically O-rings, are set in the confronting surfaces to form several seal barriers. Typically the material inside is stabilized by concrete, but even so radioactive material is quite active. In fact the vessels are often formed with cooling fins for the figuratively and literally hot contents.
In order to monitor whether any of the seals has failed, German patent document No. 2,905,094 filed Feb. 10, 1979 with no priority claim by Henning Baatz proposes a system wherein the vessel is formed with several passages that open between the seals. Such a vessel can be pressurized with a tracer gas, or the chambers themselves can be thus pressurized. In this manner a sniffer connected to the other end of any of these passages can detect the presence or absence of this tracer gas as well as any leaked radioactivity. In addition a pressure reading of each of these chambers can often provide valuable information.
To this end the upper rim of the vessel is formed with recesses in which the valves for the other ends of the passages open. Thus this rim must be provided with a safety cover to protect these elements. The provision of this extra cover, normally in addition to the above-described cover and a so-called second safety cover overlying it, represents an noticeable manufacturing expense. In addition the passages in the vessel, which may weigh over a ton empty, must be made in situ, that is they cannot be easily conveyed to a shop. This again adds to costs.